Contamination of rivers, underground water and so forth caused from various industrial effluent, sewage sludge, excrement sludge have raised a critical social problem. Therefore, techniques for the waste water treatment have attracted keen interest.
Under the foregoing circumstances, a variety of waste water treatment techniques have been investigated. Flocculating treatment using a polymer flocculating agent is one of the foregoing techniques.
For example, in Japanese Patent Laid-Open No. 58-216707 and Japanese Patent Laid-Open No. 2-298400, a technique has been suggested which uses sodium polystyrene sulfonate to serve as a waste water treatment material. According to the foregoing disclosures, an effect of flocculating the various waste water has been reported.
Available sodium polystyrene sulfonate, however, suffers from an unsatisfactorily small particle size of generated flocs, excessively low settling velocity and filtration velocity and unsatisfactory large water content in the dehydrated sludge (cakes) and turbidity of the filtered solution. From the foregoing viewpoint, the performance of the foregoing sodium polystyrene sulfonate is unsatisfactory.
On the other hand, treatment of sludge, such as sewage sludge, excrement sludge or various industrial waste water, has been performed by adding a flocculating agent (a dehydrating agent) to the sludge to flocculate suspended substances so as to produce flocs. Then, sludge from which the flocs have been obtained is dehydrated by a belt-press dehydrating machine or a filter-press dehydrating machine so as to separate the sludge into a solid component (a cake) and water. The foregoing sludge treatment must realize a low water content in the flocs and cakes produced in the foregoing process, a high filtration velocity required in the process for filtering sludge and a satisfactory separation characteristic between the filter cloth and the cakes.
As the dehydrating agent for use in the foregoing sludge treatment, cation polymer flocculating agent, such as polymer of quaternary salt of dimethylaminoethyl acrylate or dimethylaminoethyl methaacrylate or copolymer of quaternary salt of dimethylaminoethyl acrylate or dimentylaminoethyl methacrylate and acrylamide is solely employed. As an alternative to this, the cation polymer flocculating agent and an anion polymer flocculating agent are combined with each other.
A variety of polymers have been suggested as the anion polymer flocculating agent. For example, polyacrylic acid has been suggested in Japanese Patent Laid-Open No. 58-216707, while sodium polystyrene sulfonate having an average molecular weight of 1000 to 1,000,000 has been disclosed in Japanese Patent Laid-Open No. 2-298400.
However, sole use of the cation polymer flocculating agent suffers from a high water content of the produced flocs and, therefore, the realized dehydration characteristic is unsatisfactory. If the cation polymer flocculating agent and the anion polymer flocculating agent are combined with each other such that sodium polyacrylate or sodium polystyrene sulfonate having the average molecular weight of 1000 to 1,000,000 is employed as the anion type flocculating agent, there arises a problem of a low filtration velocity of sludge and an unsatisfactory processing efficiency.